Error E129

Safety precautions:

1. If you smell gas, do not operate electrical switches, do not use phones inside the property — leave the building immediately and call your local gas emergency number (in the UK call 0800 111 999) or the emergency service relevant to your country.

2. Before touching the boiler controls, switch off power at the isolator if you need to access controls for a reset. Do not open the boiler casing or attempt internal repairs unless you are a qualified Gas Safe engineer.

3. Ventilate the boiler area and ensure there are no flammable materials nearby.

Initial homeowner checks (safe, do these first):

1. Note the exact error (E129), how often it appears, and whether it happens on central heating, hot water, or both. Take a photo of the display and any sequence of events — the engineer will find that useful.

2. Try a controlled reset: follow the boiler manufacturer’s reset procedure (usually a reset button held for 3–5 seconds). If the error clears and the boiler runs normally for a sustained period, monitor it closely. If E129 returns quickly or repeatedly, do not repeatedly reset — this masks the fault and wastes time.

3. Check other gas appliances: can you light a gas hob or cooker? If no other gas appliance will light, there may be a gas supply issue — contact your gas supplier or a registered engineer.

4. Check the condensate pipe (if fitted) for freezing or blockage in cold weather. A blocked condensate can cause related faults. Thaw gently with warm (not boiling) water or towels; do not use open flames or excessive heat.

5. Make sure external vents, air inlets and the flue terminal are clear of debris, nests, snow or ice. Restricted air/ventilation can affect combustion.

6. Confirm mains power is present and the boiler has not lost electrical supply; under-voltage or intermittent supply can cause ignition problems.

If the basic checks do not clear the fault, prepare for a professional visit. Information to record for the engineer:

1. Exact model and serial number of the boiler, the operating mode when the fault occurred (heating or hot water), how many attempts before lockout, and whether the fault started after any work, service or power interruption.

2. Any recent changes: new thermostat, plumbing work, or service history.

Diagnostic and fix steps an engineer should perform (do NOT attempt unless qualified):

1. Inspect flame detection system: check condition and correct positioning of the ionisation/flame probe and its wiring. Clean or replace the electrode if degraded; check the flame sensing circuit for correct continuity and correct reference earth.

2. Check ignition and gas valve operation: verify the ignition transformer/igniter, ignition lead, and gas valve energising sequence. Measure gas manifold pressure and confirm gas supply under load.

3. Check wiring and connectors: inspect plug connections to the gas valve, electrode, PCB and fan for corrosion, loose pins or broken wires; repair or replace as necessary.

4. Check fan and flue components: ensure the fan runs correctly and the flue is not obstructed; verify flue thermostat/thermistor operation if fitted.

5. Review PCB logs and error history: the boiler PCB may show related fault codes that identify intermittent faults or components failing during calibration. Replace PCB only if confirmed faulty after proper diagnostics.

6. Run combustion and safety tests after repair: carry out a full combustion check, confirm stable flame signal over the control range, and verify all safety interlocks and sensors are working correctly.

Final notes and when to call a professional:

1. Because E129 involves flame detection and gas/ignition systems, you should call a Gas Safe registered engineer if the error persists after the simple homeowner checks. Do not attempt internal repairs, gas pressure tests, or component replacements yourself.

2. If the boiler locks out repeatedly, avoid repeatedly resetting it — record occurrences and wait for the engineer. Provide the engineer with photos and notes to speed diagnosis.

3. If you have any doubt about safety at any point (smell gas, intermittent ignition, unusual smells or noises), isolate the boiler power, turn off the gas supply at the meter or appliance isolation, ventilate and contact emergency gas services and a Gas Safe engineer immediately.